This invention relates to computer radiography in general and more partricularly an improved detector arrangement for use in computer radiography equipment.
An arrangement for the production of X-ray pictures by computer radiography which includes a camera containing a storage plate, and an image reader to scan the photo-stimulated luminescence of the storage plate line by line with a laser is known. After transmission of the light signals obtained during scanning with light waveguides, each being coordinated with one picture element of a picture line, the light signals are converted in a detector into electrical signals which are fed to an associated image processing computer.
Thus, in digital computer radiography the image obtained by X-ray irradiation is stored on what is called a storage plate and later converted into digital electrical signals by an image reader and then made visible on a picture screen. In the storage plate, the X-rays are converted into electrical charges. The storage plate contains, for this purpose, a luminous substance cast into an organic binder and applied to a transparent carrier, generally a foil. The image reader transforms the areal pattern of different X-ray intensities into corresponding electrical signals which are fed to an image processor via an analog-to-digital converter. After conversion into analog signals again, the image can be fed to the picture screen.
Stimulated by high-energy rays, in particular a red laser, the storage plate is scanned, and the photo-stimulated luminescence is fed point by point sequentially, each by means of a light waveguide, to a common photomultiplier and to an amplifier. The laser beam is focussed on the storage plate by means of an optical system and deflected over the line by means of a rotating mirror. The storage plate is shifted stepwise relative to the laser beam fan so that the entire image is scanned line by line by the laser beam (Radiology, Vol. 148, Sept. 1983, pages 833 to 838).
In another known arrangement, the rays of a common radiation source for the photo-stimulating luminescence are transmitted by means of a multiplicity of light waveguides, and this radiation is relayed with magneto-optical switches in the picture line by a light switching array (LISA) (Journal of Non-Crystalline Solids 47, 2 (1982) pages 227 to 238).
The stored, areal image pattern of the X-ray picture is converted in the image reader into a sequence of electrical signals. The image reader, thus, acts as an areal transformer. The photo-stimulated radiation can be measured "in reflection" on the side facing the scanning laser beam, or "in transmission" on the reverse side of the storage plate. The red light of the laser releases blue light in the individual picture elements of the luminous storage substance. Generally, so-called luminous phosphors, e.g., europium-activated barium fluoride compounds BaFX:Eu, serve as the luminous storage substance. Therein, X are suitable additives such as chlorine, bromine or iodine. The use of chlorine as an additive is well suited because it is relatively easy to process, although the duration of its luminescence of approximately 7 .mu.s is relatively long as compared to that of bromine of 0.8 .mu.s and that of iodine of only 0.6 .mu.s, for instance.
If, in scanning, an afterglow of the picture elements occurs whose duration corresponds to the scanning time for several adjacent picture elements, the radiation signals of the momentarily stimulated picture element and the afterglow of the previously stimulated picture elements are superposed in the receiver. This falsifies the reproduction of the radiation distribution, and resolution and dynamics are impaired. If, in scanning, the stimulating laser light is scattered, adjacent picture elements can be stimulated to luminesce in addition to the momentarily measured picture element. The emitted signals of all picture elements affected are, in turn, subject to scattering and are received and superposed by the same receiver. This adds to the worsening of resolution and dynamics of the radiation image.
Therefore, it is an object of the present invention to provide an arrangement of the kind described above in which an unfavorable influence of afterglow and, if applicable, also of the scattered light of the picture elements upon the reproduction of the stored X-ray picture is made impossible.